1. Technical Field of the Invention
The present invention relates generally to an oxygen sensor output correction apparatus for an internal combustion engine which is designed to cut a supply of fuel into the engine to place an oxygen sensor in the atmosphere or fresh air within an exhaust pipe and sample an output of the oxygen sensor to determine a correction factor for use in correcting or compensating for an error in the output.
2. Background Art
There are various techniques for installing an oxygen sensor in an exhaust pipe of an internal combustion engine and sampling an output therefrom indicating the concentration of oxygen contained in exhaust emissions to perform given control tasks for improving the quantity of the emissions. For example, engine control systems for gasoline engines are known which are designed to sample the output of the oxygen sensor to determine the air-fuel ratio of an air-fuel mixture charged into the engine and bring it into agreement with a target value in an air-fuel ratio feedback control mode for controlling the quality of exhaust emissions. Engine control systems for diesel engines are also known which are designed to control an operation of an EGR (Exhaust Gas Recirculation) valve to enhance the ability of a catalyst to clean the exhaust emissions.
Usually, typical oxygen sensors have the problem with an error in an output therefrom arising from the individual variability in operation or aging thereof. In order to alleviate such a problem, there have been proposed techniques for sampling an output of the oxygen sensor during a fuel cut event in which the engine undergoes a fuel cut for correcting or compensating for an error in the output of the oxygen sensor when the engine is undergoing no fuel cut (which will also be referred to as an under-atmosphere correction mode below) based on the fact the fuel cut during running of the internal combustion engine will cause the inside of the exhaust pipe to be placed in the atmosphere.
Japanese Patent First Publication No. 2007-32466 teaches an internal combustion engine control system designed to execute the under-atmosphere correction mode when a change in output of the oxygen sensor per unit time has dropped below a preselected threshold value during the fuel cut event or when an integrated amount of intake air charged into the engine after the start of the fuel cut event has increased above a preselected threshold value.
The engine control system, as taught in the above publication, is designed based on the fact that the combustion gas is usually placed with fresh air within the exhaust pipe after the start of the fuel cut, thus ensuring the accuracy in compensating for an error in an output from the oxygen sensor using an output therefrom, as sampled after the start of the fuel cut.
The inventors of this application measured an actual change in concentration of oxygen within the exhaust pipe after the start of the fuel cut and found that the time the exhaust gas takes to be replaced with the fresh air completely, that is, the time consumed until the concentration of oxygen in the exhaust pipe is brought into agreement with that (i.e., 20.9%) in the fresh air is long, which may cause the concentration of oxygen in the exhaust pipe not to reach that in the fresh air until completion of the fuel cut, thus resulting in an decrease in accuracy in the under-atmosphere correction mode. The inventors observed that the concentration of oxygen in the exhaust pipe may take ten (10) or more minutes to reach that in the fresh air (i.e., 20.9%) after the start of the fuel cut.